There were many flaws in the study including that women may have already had breast cancer when they were enrolled in the study and that they were at increased risk of dying within three years which was 'biologically implausible', the researchers said.

Writing in the Journal of Family Planning and Reproductive Health Care Prof Samuel Shapiro from the University of Cape Town and others said: "The name 'Million Women Study' implies an authority beyond criticism or refutation.

"Here we conclude that the evidence in the Million Women Study was indeed unreliable. There were defects in the study design, and the findings did not adequately satisfy the principles of causation.

"HRT may or may not increase the risk of breast cancer but the Million Women Study did not establish that it does."

The authors said inviting women to join the study would in itself have increased the number already aware of breast lumps or precancerous changes, leading to higher numbers of cancers being detected (detection bias), they added.

This is borne out by a higher rate of cancers among the study participants than in the general population, irrespective of whether or not they used HRT.

The authors also argued that crucial data was often also missing.

Prof Shapiro wrote: "Yet the validity of any study is dependent on the quality of its design, execution, analysis and interpretation. Size alone does not guarantee that the findings are reliable.

"The Million Women Study was an observational study, and it has the attendant problems and uncertainties intrinsic to such studies.

"If the evidence was unreliable, the only effect of its massive size would have been to confer spurious statistical authority to doubtful findings."

Prof Shapiro and the other authors declared 'competing interests' in the research paper, in that they had all acted as consultants for pharmaceutical companies that make HRT, currently or in the past.

Dr Susie Jennings, Senior Policy Officer at Breakthrough Breast Cancer, says: “Many studies have shown that HRT increases the risk of breast cancer and this study does not change that. It does suggest the risks may, in some instances, have been slightly overstated.

“However, we know that the risk of breast cancer starts to decrease as soon as a woman stops taking HRT.

“Anyone who has concerns about HRT should speak with their doctor.

“We hope the Breakthrough Generations Study, which is following 110,000 UK women over 40 years, will provide further clarity on this issue.”

Hazel Nunn, head of health information at Cancer Research UK, said: “Women shouldn’t be unduly worried by this latest review of HRT and breast cancer risk. HRT can be an effective short-term treatment for menopausal symptoms – women taking it should try and use the lowest dose possible for as short a time as they need it.

“If you are considering starting or stopping HRT, or using it for a long time, you should discuss it with your doctor. The issues are different for every woman and your doctor will be able to help you weigh up the benefits and risks of different types of HRT and make the right choice based on your own circumstances.

“In the UK, HRT use has been falling since the start of the Millennium. Studies suggest that in 2005 there were 1,400 fewer cases of breast cancer in the UK among women aged 50-59 than would have been if there had been no drop in HRT use.”

The co-authors of the Million Women Study refuted the claims.

In a joint statement, Professor Valerie Beral and Professor Richard Peto, from Oxford University, said: “HRT is one of the most important causes of breast cancer in the world and women can easily change their risk by stopping.”

Prof Dame Valerie Beral added: "This paper is a re-statement of views held by many consultants to HRT manufacturers (as these authors are) attempting to dispute evidence about the adverse effects of HRT. The issues raised are not new and have been refuted previously.

"The authors omit to say that Million Women Study findings, of an increased risk of breast cancer in users of HRT, especially of oestrogen-progestagen combinations, have been replicated in over 20 other studies. The totality of the worldwide evidence is now overwhelming.

"In line with the findings from these studies, the recent large decrease in HRT use has been followed in many countries by a nationwide decline in the incidence of breast cancer."